1. Field of the Invention
This invention relates to a resin composition and a printed circuit board using the same, and more particularly to a resin composition capable of providing a printed circuit board having improved appearance and color tone.
2. Disclosure of Related Art
As a method of producing a printed circuit board, there is an additive method. This additive method is a method wherein an adhesive for electroless plating is applied onto an insulating substrate of glass-epoxy resin or the like to form an adhesive layer, and the surface of the adhesive layer is roughened, and a plated resist is formed on the roughened surface, and then a metal for a conductor circuit is adhered thereto through electroless plating.
According to this method, the conductor circuit is adhered to the toughened adhesive layer through plating or the like, so that an excellent joining property can be ensured therebetween to prepare a printed circuit board having an excellent reliability.
Therefore, the adhesive for electroless plating used in the above method is necessary to have a function that the surface after the curing can be roughened. For this purpose, there is adopted an adhesive formed by adding a soluble filler to an epoxy resin as a resin matrix (see JP-A-5-18476). Such an adhesive made from the epoxy resin is excellent in the heat resistance as compared with an adhesive made from rubber or the like and can improve the heat resistance of a printed circuit board.
Furthermore, there is proposed a photosensitive resin composition formed by giving a photosensitive group to the epoxy resin as-the resin matrix (see JP-A-4-55555). When using such a composition, the occurrence of strain cracking produced in the heat-curing of the adhesive is effectively prevented by the curing through light exposure, or the formation of via-hole on an interlaminar insulating layer in a so-called built-up multi-layer printed circuit board suitable for high densification can easily be made by light exposure and development.
However, the aforementioned epoxy resin or photosensitive resin composition has a light permeability, so that when it is used as a resin matrix of an adhesive for electroless plating, an internal circuit can be seen from the exterior and hence an appearance and a color tone of the printed circuit board are undesirably degraded.
For this end, the adjustment of color tone in the printed circuit board has hitherto been carried out by a so-called solder resist process in which a colored solder resist is printed on the printed wiring substrate. In the solder resist process, however, the resin material constituting the printed wiring substrate is discolored through oxidation by a heating treatment applied in the part mounting or the like. Such a discoloration is usually dark brown and forms a stripe pattern without showing constant coloration, so that the appearance of the printed circuit board is not yet improved even if the solder resist is printed. Particularly, since the resin layers are successively piled one upon the other and cured in the multi-layer printed circuit board of built-up structure, the degree of discoloration is remarkable and even when the colored solder resist is formed as the outermost layer, the discolored portion of the under layer is not sufficiently covered with the presence of the outermost layer, and hence the non-uniform discoloration of the underground layer is undesirably permeated.
In order to cover such a discoloration, there are a means for increasing an amount of a coloring agent to make a deeper color, a means for increasing a thickness of a colored solder resist layer and the like. However, the addition of a great amount of the coloring agent generally brings about the degradation of the insulating property, while the thickening of the resist layer brings about the increase of material cost. In case of using the photosensitive resin, the resolution of the resin undesirably lowers.